1. Field of the Invention
The present invention relates to a combustion-engined setting tool for driving in fastening elements such as, e.g., bolts or nails in an object and including a combustion chamber, a guide cylinder adjoining the combustion chamber, a drive piston displaceable in the guide cylinder in a setting direction upon combustion of a fuel gas in the combustion chamber, a metering chamber for metering a predetermined amount of the fuel gas to the combustion chamber, and an ignition device for igniting the fuel gas in the combustion chamber.
2. Description of the Prior Act
Setting tools of the type described above all have a combustion chamber in which a fuel gas is ignited. Upon ignition of the fuel mixture in the combustion chamber, a drive piston is displaced in a guide cylinder that adjoins the combustion chamber as a result of expansion of the ignited fuel mixture in the combustion chamber. With the driven piston, a fastening element, which is located in front of the piston, e.g., a nail, is driven, e.g., in a wall located in front of the setting tool or in another surface.
The fuel, which is stored in a pressure reservoir, after it has been withdrawn from the reservoir and before it is injected into the combustion chamber, should be optimally metered out by an appropriate device to insure that an optimal amount of the fuel/air mixture is fed into the combustion chamber. An optimal amount of fuel/air mixture is necessary to insure optimal combustion of the fuel mixture in the combustion chamber.
For an effective operation of a combustion-engined tool, it is necessary that a drive force for driving the piston, which is generated upon ignition of the fuel/air mixture, be the same for each setting process. Because an available amount of oxygen for each combustion process depends to a great extent on the air pressure and the air humidity, the necessary amount of the oxygen varies greatly with changes in these parameters, up to 40% in an extreme case. In order to compensate these variations, there exist metering devices which insure, to greater or lesser degree, a uniform power output of a setting tool.
There exist many methods and metering devices which provide a uniform metering. At a volumetric metering of liquid or liquefied fuels when a fixed metered volume is produced, first, the fluid fuel is fed into a metering valve in which a predetermined to-be-metered volume is preset. With an appropriate actuation device, the fluid fuel contained in the metering valve is fed into a combustion chamber. A drawback of this method consists in that the preset to-be-metered volume of the metering valve is fixed, which does not permit to vary the metered amount of fuel. Therefore, the fuel amount cannot be adapted to variable environmental conditions or operating conditions. Thus, a larger amount of fuel cannot be delivered at low environmental temperatures. Likewise, a smaller amount of fuel cannot be delivered at high temperatures. The set volume of the metering valve can be only determined as a compromise between extreme values and the environmental temperatures and, as a result, an optimal combustion cannot be achieved either at low temperatures or at high temperatures. Further, no power output control of a power tool is possible by varying the amount of fuel.
A volumetric metering with a preset volume is also used for metering of gaseous fuels. With this method, the fuel, after it is withdrawn from the reservoir, is evaporated. The metered-out volume at a gaseous metering is about from 100 to 300 times greater than at a fluid metering. Here, likewise, a fixed metered-out volume is a drawback. The fuel is evaporated before it is fed into a metering chamber by throttling. The necessary evaporation heat is communicated to an evaporator in which the liquid fuel is evaporated before it is fed into the metering chamber. The pressure in the metering chamber is always commensurated with the environmental pressure by providing the metering chamber with a bleed opening open into the environment. In this way, the change in the environmental pressure is compensated. Because a gaseous fuel is fed into the metering chamber, the density of the fuel varies, within certain limits, proportionally to the air density. Thus, the fuel amount is automatically changes with variations in the environmental temperature and is automatically adapted to the environmental pressure. However, in this case, a necessary pre-condition consists in that the temperature in the metering chamber and the air temperature be substantially the same. With a hot tool, this is rarely the case. With this method also, a power output control or the compensation of other environmental conditions, e.g., air humidity, is not possible. The control of pressure in the metering chamber is not contemplated.
Also known are metering methods with which opening of metering valve, e.g., a magnetic valve, is time-controlled. With these methods, by using an electronically controlled magnetic valve, an injected amount of a fluid fuel is measured and the metering valve remains open for a predetermined time period for feeding a desired amount of fuel into the metering chamber. By time-controlling the metering valve, other parameters, which influence the fuel amount or fuel volume, can be taken into account.
A system of the type described above is disclosed in U.S. Pat. No. 6,223,963. The patent discloses a power tool in which a fuel injection circuit controls a time period within which the inlet side of the metering valve remains open. A microprocessor controls the filling period dependent on the time period, gas temperature, temperature in the combustion chamber, and the battery voltage. At an increased temperature, the filling period is reduced, and at a lower temperature, the filling period is increased. The control circuit controls the time of opening of the metering valve dependent on the environmental temperature and/or the environmental pressure.
When metering with a time-control of the opening of the magnetic valve is used for metering of fluid fuels, the fluid is initially fed to the combustion chamber in a fluid or liquid form. This means that the fuel should be evaporated either in its path from the metering device to the combustion chamber or in the combustion chamber. In particular, at cold environmental temperatures, with a lower vapor pressure and an extended evaporation, there is a danger that by the time ignition is effected too little fuel would evaporate, which reduces the power output or, basically, no ignition takes place. Also, at rapid setting processes, there is a danger that too little fuel has evaporated, the power output is reduced, and no ignition takes place.
Accordingly, an object of the present invention is a combustion-engined tool in which a uniform amount of fuel is metered in the metering chamber under changing environmental conditions.